In a variable-speed, constant-frequency (VSCF) power conversion system, a brushless, synchronous generator is supplied variable-speed motive power produced by a prime mover and develops variable-frequency AC power at an output thereof. The variable-frequency power is rectified and provided over a DC link to a controllable static inverter. The inverter is operated to produce constant-frequency AC power, which is then supplied over a load bus to one or more loads.
Usually, such VSCF systems must provide a high level of load current, and hence switches used in the inverter must be high power devices and/or be connected in parallel. High power switching devices are expensive and somewhat difficult to control. Parallel-connected power switches encounter problems due to current sharing, and hence this approach is not entirely satisfactory as an alternative.
Heintze, U.S. Pat. No. 3,943,429, Honbu et al., U.S. Pat. No. 4,549,258 and Mizoguchi, U.S. Pat. No. 4,802,079 disclose polyphase inverters wherein each phase comprises first and second subinverters having outputs interconnected by a reactor such as an interphase transformer or a choke. A mid-tap of the reactor is coupled to a load. The switches of such inverters are subjected to only a fraction of the load current, and hence devices of smaller current rating can be used. Also, the reactor eliminates problems arising from current sharing.
As is known, a generator can be operated as a motor in a starting mode to convert electrical power supplied by an external AC power source into motive power which may in turn be provided to the prime mover to bring it up to self-sustaining speed. In the case of a brushless, synchronous generator having a permanent magnet generator (PMG), an exciter portion and a main generator portion mounted on a common shaft, it is necessary to provide power developed by an external power source at a controlled voltage and frequency to armature windings of the main generator portion and to provide field current to the main generator portion via the exciter portion so that the motive power may be developed.
Dhyanchand, co-pending application Ser. No. 07/408,928, filed Sept. 18, 1989 and assigned to the assignee of the instant application (Sundstrand Docket No. B03265-ATl-USA) discloses a VSCF system of the above-described type having a set of three exciter field windings which are connected in a wye configuration in a starting mode of operation. During operation in a generating mode of operation, two of the exciter field windings are connected in parallel and the remaining winding is connected in series with the parallel combination. The same windings may therefore be used in the starting and generating modes. AC power for the exciter field windings during operation in the starting mode is obtained from an external AC power source, a rectifier coupled to the external AC power source and an exciter power converter coupled to the rectifier. During operation in the generating mode, DC exciter power is obtained from the PMG, an autotransformer which steps down the PMG armature voltage level, a rectifier and a power converter which may form a part of the exciter power converter.
Shilling, et al., U.S. Pat. No. 4,743,777 discloses a starter generator system using a brushless, synchronous generator. The system is operable in a starting mode to produce motive power from electrical power provided by an external AC power source. An exciter of the generator includes separate DC and three-phase AC field windings disposed in a stator. When operating in a starting mode at the beginning of a starting sequence, the AC power developed by the external AC power source is directly applied to the three-phase AC exciter field windings. The AC power developed by the external AC source is further provided to a variable-voltage, variable-frequency power converter which in turn provides a controlled voltage and frequency to armature windings of a main generator. The AC power provided to the AC exciter field windings is transferred by transformer action to exciter armature windings disposed on a rotor of the generator. This AC power is rectified by a rotating rectifier and provided to a main field winding of the generator. The interaction of the magnetic fields developed by the main generator field winding and armature windings in turn causes the rotor of the generator to rotate and thereby develop the desired motive power.
When the generator is operated in a generating mode, switches are operated to disconnect the AC exciter field windings from the external AC source and to provide DC power to the DC exciter field winding.
Messenger, U.S. Pat. No. 3,908,161 discloses a brushless generator including three exciter field windings which are connected in a wye configuration and provided three-phase AC power during operation in a starting mode. The three-phase AC power induces AC power in an exciter armature winding which is rectified and applied to a main generator field winding. Main armature windings receive controlled AC power to in turn cause rotation of the generator rotor. Thereafter, the three exciter field windings are connected in series and provided DC excitation when operating in a generating mode.